As described in “The History of Shaped Charges” by Donald R Kennedy, the concept of shaping an explosive charge, in order to focus its energy was known in 1792. In 1884 Max von Forester conducted experiments in Germany showing that an explosive charge with a hollow cavity will focus the explosive energy and produce a collimated jet of high speed gasses along the longitudinal axis of the cavity. When this cavity is lined with a ductile metal it will produce a high speed collimated stretching jet of liquefied material capable of penetrating all known materials.
In 1888, while conducting research for the U.S Navy, at Newport R.I., Charles Munroe discovered that not only could explosive energy be focused, but lining the hollow cavity in the explosive with metal increased the penetration dramatically, the effect is commonly called the Munroe Effect. These discoveries were further studied in 1910 by Egon Neumann of Germany who conducted similar experiment's, which showed that a cylinder of explosive with a metal lined conical hollow cavity could penetrate through steel plates. The military implications of this phenomenon were not realized until the lead up to World War II.
In the 1930's flash x-ray technology was developed which allowed the in depth study of the Shaped Charge jetting process. With this new diagnostic, it was possible to take X-Ray pictures of the collapse of the liner and the resulting jet. This new diagnostic led to a more scientific and complete understanding of the Munroe principle and emphasized the power of shaped charges.
Generally, when a cylinder of explosive with a hollow conical cavity at one end is detonated at the center of the opposite end, the energy of the explosive is focused into a rod-like jet of high temperature, high pressure and high velocity gases along the axis of a conical cavity. This is an axisymmetric collapse and is generally known as the Munroe effect. The pressures created behind the detonation front in the explosive are of such magnitude that it causes the metal of the liner to liquefy and flow like a fluid. As the liner material is collapsed toward the axis of the hollow cavity, the flowing material radially converges, creating a rod-like stretching jet of high velocity, between five and ten kilometers per second.
These jets are primarily copper and will penetrate all known materials. The conventional shaped charge will give typically a hole size that is, in a semi-infinite target; could be as high as 20% of the diameter of the shaped charge. In order to achieve the greatest jet length and penetration depth, the jetting process of a shaped charge requires the liner material to reach a high temperature during collapse, which allows plastic flow of the collapsed liner material that produces a long stretching jet.
Plastic flow is accomplished by forcing the liner material under great pressures to collapse and converge radially onto the liners symmetrical axis. A typical linear or circular linear shaped charge liner has non-fluted or non-corrugated walls, is driven from only two dimensions and has insufficient convergence to cause plastic flow and high velocities, so these devices do not produce ductile stretching jets but instead produce explosively formed projectiles EFP.
Modern shaped charges are used for various purposes, such as oil field perforators, and they produce a long stretching rod-like metal jet that penetrates 4 to 8 charge diameters in steel and as much as three times deeper in masonry or rock. The average diameter of a 5 CD deep hole from these conventional shaped charges is less than 15% of the diameter of the explosive charge CD. These types of charges are designed to have long, stretching rod-like jets, primarily to penetrate the walls of a vehicle or other target, which has been the focus of a vast majority of research in this field. The small holes produced by these types of charges do not permit a follow-through device in the case of surgical destruction of a protected enclosure.
Modern shaped charges can produce a long stretching rod like metal jet that penetrates about 5 to 8 charge diameters in steel, deeper in masonry or rock. The average diameter of a five charge diameter CD through hole from these type charges is less than 15% of the explosive charge diameter. These small diameter holes made by conventional jets do not produce a hole of sufficient diameter to provide a means to deliver follow on shaped charges of equal charge diameter to the standoff needed from the bottom of a hole with the intent of making an equal size hole diameter and depth of penetration as the last charge.
There have been some specialized efforts by Halliburton to produce shaped charges other than conical type shaped charges for special purposes such as pipe cutting and anchor chain cutting. These type of charges are called linear shaped charges and use the Munroe principle to produce a thin sheet like jet with somewhat similar cutting power to the usual conical shaped charge. The liner is wide angle and the device is used against light structures such as wooden doors and thin walls. The vast majority of research and development in shaped charges over the past hundred years or so has been devoted to deep penetration in both military and commercial applications. Some efforts have been directed to increasing the internal angle of the liner and a shorter standoff.
Other devices using flexible linear shaped charges have been designed for breaching man-size holes in light walls, such as described in Wall AXE British, 1960. These line charge devices are collapsed from only two opposing directions producing a very irregular thin sheet-like jet that is unpredictable in its penetrating ability due to the lack of a simultaneous initiation along the apex of the line explosive. These line charges are limited in the thickness or toughness of the target they can address and are mainly used for light walls. Additionally, sometimes users such as police or firefighters are badly injured or killed trying to use these awkward and clumsy devices.
U.S. Pat. No. 7,753,850 places an interrupter along the jet axis inside the liner, in the flow path of the liner material. The permissible size of the interrupter for this concept can only be a small portion of the liner diameter so as to leave room for the liner to collapse. The small diameter of the interrupter does not form a large enough diameter jet to produce a full caliber hole or to hold its annular shape after it separates from the interrupter; the jet will converge into a rod and some of the precious liner length is wasted.
U.S. Pat. Publ. No. US2011/0232519 A1 shows outside and inside walls making up the circular trough of the liner. The mass of the outer wall of the liner trough, because of its greater diameter, is much greater than the mass of the inner wall. The outer wall is converging whereas the inner wall, with much less mass, is diverging; the same problem exists with the explosive quantities driving each wall of the liner. To obtain a circular or annular jet, these masses must be equal in forces when they converge on the projected axis of the liner cavity.
In steel-making, small conical shaped charges are often used to pierce taps that have become plugged with slag. Linear shaped charges, or line charges, are another type of shaped charge used in the demolition of buildings to cut through steel beams and collapse the building in a desired pattern. This type of flexible line charge creates a sheet-like jet from a two-dimensional collapse. SWAT teams and fire departments are another user of line charges, using the Munroe principle to generate high speed material for urban wall breaching and rescue. These line charges are very inefficient and difficult to initiate in a manner conducive to achieving their full potential. Very little research has been conducted in this area of shaped charge technology, and all of these applications of shaped charges would benefit greatly from a larger-diameter penetration capability.
Hole diameters in casing from these conventional charges are not greater than ½ inch in diameter. The expected perforated holes sizes can be inconsistent, varying in size to more than 50% from the target diameter. This inconsistency causes many fracturing operation issues, and small hole size limits product flow into and from the formation; if too small, the perforation will get fouled with debris and can stop flowing altogether. The hole diameter produced by a present day oil well perforator is only approximately 12% of its explosive charge diameter. Great efforts have been made over the last 50 or so years to enlarge the entry hole diameter in oil well casing without much success.
Some effort has been made with placing a conventional shaped charge ahead of the projectile in order to create a pilot hole in the rock; however, only a small gain in depth of penetration is achievable with this method because of the very small hole diameter produced by a conventional shaped charge. The hole diameter made by a conventional shaped charge jet is small, on the order of one-tenth the diameter of the explosive charge forming the jet, and it penetrates approximately 6-8 times the diameter of the charge in steel (more in rock or masonry).
There is clearly a need for innovation in this industry to have a shaped explosive device that produces a combination of a forward rod and rearward flattened Spade shaped stretching jet.